In U.S. patent application Ser. No. 13/713,744, imaging array devices with metal-oxide thin-film-transistor (MOTFT) pixel readout circuit over PIN photodetector array were disclosed. Similar device architecture and process flow can also be used for making display devices in which each pixel comprises a MOTFT pixel driver circuit constructed with the MOTFT positioned over a light emitting display element made in PIN stack formation.
Recently, the emission efficiency of GaN LEDs has been improved rapidly to over 150 lm/W and it has become an ideal light source for projector displays due to its long operation lifetime compared with the gas discharge lamp. However, using an LED to replace the gas discharge lamp is limited by the size of the LED. It is hard to make a large uniform LED source that can work with a flat liquid crystal display (LCD) or digital micro-mirror display (DMD) engine. The major issues are current crowding and the loss of homogeneity as LEDs get larger. Here we propose to use an array of LEDs to overcome these issues. Because of the patterning into individual small LEDs (<100 microns), there are no issues of current crowding and inhomogeneity in driving the LEDs. The LED array consists of M×N LEDs where each LED can be driven independently to create the image. Each LED is very small and efficient without current crowding. To generate 5000 lm in a projector, assuming efficiency of 100 lm/W, one needs 50 W and 16 A of current (3V for GaN based PIN emission junction). Using current density of 10 A/cm2, only an effective emission area of 1.6 cm2 is needed. So the emitter array can be made in a very small area. Assuming a moderate fill factor (emitting area within each display pixel) of 30%, less than 5 cm2 of GaN area is needed. The 10 A/cm2 driving current is well below what the GaN LED is capable of (>100 A/cm2). So one has the capability to increase the luminance by another factor of 10; i.e., 50000 lm. When such active matrix LED display (AMLED) is used in a projector for a commercial Movie Theater or for outdoor displays at brightness of 500 cd/m2, over 100 square meter screen can be covered. Such AMLED is, thus, capable of making an extremely high power projector for indoor and outdoor display applications.
A key challenge for such high power density projector is to dissipate the heat. The heat removal through thermal conduction alone is difficult because of the other requirements, e.g., the heat spreading material in such a device has to be transparent. The best transparent heat spreader is the SiC or GaN substrate that is used to grow the GaN LED. Sapphire is also transparent, but its heat spreading capability (23 W/m-K) is much smaller than SiC (120 W/m-K) or GaN (130 W/m-K). GaN has the additional benefit of lattice matching and enables better quality crystal growth. GaN can also be grown on Si, but it is not transparent and the substrate has to be removed in order to optimize its optoelectronic performance.
Another challenge is to provide an active matrix to drive the large array of M×N individual LEDs. There are two solutions to this problem. One solution is to make m×n connections to Si driver chips by solder bumps. However, the solder bump yield is challenging to make the millions of connection and, in addition, there is another serious problem in using Si chip right on top of the GaN LED array. The high temperature of the GaN LED junction may cause the Si integration circuit (IC) to malfunction. The leakage current of Si field effect transistors (FET) increases rapidly with temperature because of its lower bandgap (1.1 eV). To make the active matrix work for such small pixel size, the FET leakage current has to be very small due to a limited size of storage capacitance. A FET based m×n array on silicon wafers is thus difficult for such high power density, high temperature applications.
The other solution is to make an active matrix driving circuit directly on GaN LED wafers using TFT technology with a higher bandgap semiconductor channel, as disclosed in detail in this invention. The current-voltage dependence and the “off” current of a high bandgap MOTET is not strongly dependent on temperature and can be kept very small at temperatures even above 100° C. (Gang Yu et. Al., Symposium Digest of Society of Information displays, Vol. 43, p.1123 (2012); Gang Yu, et al., The 14th International Meeting of Information Displays, Daegu, Korea, Aug. 25-29, 2014). Also it can be made directly on top of a GaN LED and there is no need to use the low yield solder bump to make the millions of connection.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a high pixel density, active matrix LED array for high power projector display.
It is another object of the present invention to provide an AMLED display with MOTFT pixel driver circuits on top of a PIN LED array without a solder bonding layer.
It is another object of the present invention to provide an AMLED display with MOTFT pixel driver circuits with low leakage current, low storage capacitor and a large operation temperature range.
It is another object of the present invention to provide an AMLED display with improved thermal conduction, improved current density and improved output optical power.
It is another object of the present invention to provide a new and improved process for fabricating an active matrix light emitting array with a metal-oxide thin film transistor (MOTET) array.
It is another object of the present invention to provide a new and improved process for fabricating AMLED arrays using a fewer number of process steps.
It is another object of the present invention to provide a new and improved process for fabricating AMLED arrays with high pixel counts and high pixel density for large screen projectors for indoor and outdoor applications.
It is another object of the present invention to provide a new and improved process for fabricating an AMLED array without a solder bond layer and with improved operational stability.
It is another object of the present invention to provide a new and improved process for fabricating AMLED displays with more pixels and with smaller pixel pitch.